A relatively recent advance in design of circuit interrupters has been the so called "self-extinguishing" puffer interrupter. Here the pressure raising function of the electrical arc, due to the thermal energy released by the arc, is used to provide a source of high-pressure gas which is released through the arcing region to extinguish the arc. Those skilled in the art know that high-temperature arc extinguishing gas has a lower density which promotes ionization and has a relatively low insulating capability. In other words, higher temperature fluid has a higher conductivity and a lower capacity for extinguishing an arc. Thus, in a "self-extinguishing" puffer interrupter, large-capacity current interruption is difficult to obtain if the volume of fluid is kept constant. This limitation tends to off-set the benefits gained by being able to use a prime mover which is smaller in size than that used in an ordinary puffer interrupter. Several patents have been granted for inventions the purpose of which is to improve the arc extinguishing capability during high current carrying conditions: U.S. Pat. No. 4,221,943; U.S. Pat. No. 4,225,762; U.S. Pat. No. 4,239,949; U.S. Pat. No. 4,242,550; U.S. Pat. No. 4,243,860; U.S. Pat. No. 4,253,002; and EPC No. 00 19 806 (or U.S. Pat. No. 4,445,020). However, this effort has been largely concerned with fault interrupters (i.e., 2000 amp. load current could have a fault current of 20,000 amps). Load interrupters (i.e., 2000 amp load rating with interruption required under any load between 0 and 2000 amps) represent a separate and distinct problem to those skilled in the art.
From the foregoing it should be clear that high-pressure is relatively easy to obtain in a self-extinguishing puffer interrupter when the arc current is large (i.e., fault current) and that, as the arc current decreases, the pressure necessary to cause interruption becomes more difficult to achieve. In other words, when the current is relatively small (i.e. low laod), the pressure produced within the chamber surrounding the arc is generally insufficient to produce interruption (i.e., the volume is fixed and the thermal energy released is small). Several inventions have been patented whose purpose is to provide adequate arc extinguishing capability in the low current range: U.S. Pat. No. 4,259,555; U.S. Pat. No. 4,270,034; and U.S. Pat. No. 4,327,263. Examination of these patents and those previously cited will show that a satisfactory solution to the problem of providing a self-generating or self-extinguishing gas flow load interrupter which performs satisfactory under both full load current and low load current ranges has not yet been achieved. An inexpensive, relatively simple design for a load interrupter, which performs satisfactorily in both the high and the low current ranges, will be a welcome addition to the art.